Fix tryPeek implementation to not consume byte

Author: Sergey Mashkov

kotlinx-io-js/src/main/kotlin/kotlinx/io/core/IoBufferJS.kt

@@ -826,7 +826,6 @@ actual class IoBuffer internal constructor(
         val writePosition = writePosition
         if (readPosition == writePosition) return -1
 
-        this.readPosition = readPosition + 1
         return i8[readPosition].toInt() and 0xff
     }
 

kotlinx-io-jvm/src/main/kotlin/kotlinx/io/core/IoBufferJVM.kt

@@ -659,9 +659,12 @@ actual class IoBuffer private constructor(
         }
     }
 
+    /*
+     * Returns next byte (unsigned) or `-1` if no more bytes available
+     */
     actual final override fun tryPeek(): Int {
         val bb = readBuffer
-        return if (bb.hasRemaining()) bb.get().toInt() and 0xff else -1
+        return if (bb.hasRemaining()) bb.get(bb.position()).toInt() and 0xff else -1
     }
 
     actual final override fun peekTo(buffer: IoBuffer): Int {

kotlinx-io-native/src/main/kotlin/kotlinx/io/core/IoBufferNative.kt

@@ -907,7 +907,6 @@ actual class IoBuffer internal constructor(
         val writePosition = writePosition
         if (readPosition == writePosition) return -1
 
-        this.readPosition = readPosition + 1
         return content[readPosition].toInt() and 0xff
     }
 

src/test/kotlin/kotlinx/io/tests/BytePacketReadTest.kt

@@ -231,7 +231,9 @@ class BytePacketReadTest {
             writeByte(2)
         }.use { pkt ->
             assertEquals(1, pkt.tryPeek())
+            pkt.discardExact(1)
             assertEquals(2, pkt.tryPeek())
+            pkt.discardExact(1)
             assertEquals(-1, pkt.tryPeek())
         }
 
@@ -246,6 +248,7 @@ class BytePacketReadTest {
 
         ByteReadPacket(segment1, pool).use { pkt ->
             assertEquals(1, pkt.tryPeek())
+            pkt.discardExact(1)
             assertEquals(-1, pkt.tryPeek())
         }
     }
@@ -261,4 +264,4 @@ class BytePacketReadTest {
         }
     }
 
-}
+}